If the stories contained in a book are not true, there's really nothing for them to be but fiction, regardless of whether or not some people believe otherwise.

Parables are neither truth nor fiction.

Ditto a great deal of poetry.

Also, there's quite a bit of ancient writing, both religious and otherwise, in which, unless somebody actually comes up with a working time machine, we're never going to know the degree of factual accuracy involved. It's often hard enough to figure out, in our massively documented current society, which description of what happened only a few years ago is accurate.

-- some of this may depend on how one's defining "fiction", of course. If "fiction" is defined as "everything that can't be mathematically or historically proven to be accurate in detail", then what crocoduck_hunter said above is not only true but a tautology. "Fiction" in the sense in which Harry Potter is fiction strikes me as having a different meaning, just as "lie" in the sense of "deliberate falsehood meant to deceive" has a different meaning that "fiction" in the Harry Potter sense.